Because I had time
OK, here is 61.51 (e) ….
(e) Logging pilot-in-command flight time.
(1) A sport, recreational, private, or commercial pilot may log pilot-in-command time only for that flight time during which that person—
(i) Is the sole manipulator of the controls of an aircraft for which the pilot is rated or has privileges;
(ii) Is the sole occupant of the aircraft; or
(iii) Except for a recreational pilot, is acting as pilot in command of an aircraft on which more than one pilot is required under the type certification of the aircraft or the regulations under which the flight is conducted?
(2) An airline transport pilot may log as pilot-in-command time all of the flight time while acting as pilot-in-command of an operation requiring an airline transport pilot certificate.
(3) An authorized instructor may log as pilot-in-command time all flight time while acting as an authorized instructor.
(4) A student pilot may log pilot-in-command time only when the student pilot—
(i) Is the sole occupant of the aircraft or is performing the duties of pilot of command of an airship requiring more than one pilot flight crewmember;
(ii) Has a current solo flight endorsement as required under §61.87 of this part; and
(iii) Is undergoing training for a pilot certificate or rating.
Furthermore in FAA Notice 95-11[FR] …
As noted in Notice No. 95-11[FR], there has been a distinction between acting as pilot in command and logging of pilot-in-command time. "Pilot in command," as defined in part 1, "means the pilot responsible for the operation and safety of an aircraft during flight time." Section 61.51 is a flight-time logging regulation under which: (1) pilot-in-command time may be logged by someone who is not pilot in command as defined in part 1 (e.g., when the pilot is the sole manipulator of controls of an aircraft for which the pilot is rated but is not the pilot in command as defined in part 1); and (2) pilot-in-command time may not be logged by someone who is the actual pilot in command as defined in part 1 (e.g., when the pilot acting as pilot in command on which more than one pilot is not required under the type certification of the aircraft or the regulations under which the flight is conducted is not the sole manipulator of the controls of the aircraft, and the pilot who is the sole manipulator of the controls is logging that time as pilot-in-command time).
Furthermore (selected entries) from the FAA Chief Counsel Opinions …
Pilot In Command (PIC)
If the pilot is the sole manipulator of the controls and he or she is receiving instruction, that pilot may log the same flight time as both pilot-in-command (PIC) time and training time. However, the time may only count once towards total time. (5-17-01).
A pilot may log pilot-in-command flight time while they are the sole manipulator of the controls of an aircraft for which they are rated, the sole occupant of the aircraft, acting as PIC on an aircraft on which more than one pilot is required under the type certificate or the regulations governing the flight, or while acting as pilot in command of an operation requiring an ATP certificate (ATP certificate holders only). (8-21-00).
If more than one pilot is required under the type certificate of the aircraft or the regulations governing the flight then the acting pilot in command may log PIC flight time for the entire flight even though he or she may not manipulate the flight controls, and the second in command may log PIC flight time for all of the flight time while he or she is the sole manipulator of the controls. In addition, if the regulations require the pilot in command to hold an ATP for that operation, even if more than one pilot is not required under the type certificate of the aircraft or the governing regulations, then the acting pilot in command may log PIC flight time for the entire flight and the second in command may log PIC flight time for all of the time while he or she is the sole manipulator of the controls. (8-21-00).
Your question concerning 61.51 centers on the sentence “for which the pilot is rated.” You ask “if the airplane in question requires a type rating (for example, KC-135 or B-707), does a pilot have to possess the type rating for that aircraft before he can log PIC time during that portion of the flight during which he is the sole manipulator of the controls? Or, to the contrary, are possession of a private pilot certificate and merely being the sole manipulator of the controls sufficient to log PIC time in that aircraft?" Under 61.51, a private or commercial pilot may log as PIC time only that flight time during which he is the sole manipulator of the controls of an aircraft for which that pilot is rated. "Rated", as used under 61.51, refers to the category, class, and type as appropriate. Therefore, pilots must be appropriately rated for the aircraft, as the term is defined above, before they may log PIC time under Part 61. The possession of a private pilot certificate and merely being the sole manipulator of the controls of an aircraft is not necessarily sufficient to log PIC time. (7-30-92).
Under 61.51, concerning the logging of PIC time, the sentence "when acting as pilot-in-command of an aircraft on which more than one pilot is required under the type certification..." does not mean that a pilot, not in possession of the type certificate for that aircraft, can nonetheless act as PIC during the portion of the flight that he is the sole manipulator of the controls and therefore log PIC time for that portion of the flight. (7-30-92).
The second in command, who is the sole manipulator of the controls of an aircraft for which he is rated may log that time as PIC flight time under 61.51, in order to use that experience to meet the requirements for a certificate or rating or recent flight experience requirements of Part 61. Also, if the pilot is acting as second in command of an aircraft on which more than one pilot is required by the FAR, he may also log that time as second-in-command time under 61.51. Of course, one hour logged under both PIC and SIC time adds up to only one hour total flight time. It should be recognized that some time may be logged as PIC time for some purposes, but not for others. For example, the 100 hours of PIC time included in 135.225(d) refers to time when the pilot actually served as PIC, not when he was the SIC manipulating the controls. (12-5-84).
So if you ask a dozen forum readers …. you will get a dozen different answers. My advice is to be conservative. If you are an FO at a Major, why would you ever log PIC when you are SIC. You already have enough time as a PIC to get where your are. If you are typed in your jet and you are the sole manipulator, and you want to log PIC, that’s up to you. But why would you? Most often the Captain is always logging PIC … would you tell him not to log PIC on your leg? Investigate the rules for yourself and be informed.